The present invention relates to cardiac pacemakers, and more specifically to evaluation of myocardial performance in connection with cardiac pacemakers.
Cardiac pacemakers play an important role in the treatment of patients suffering from heart failure. Pacemakers can be single-chamber or multi-chamber pacemakers. A single-chamber pacemaker delivers pacing pulses to one heart chamber to maintain a normal heart rhythm. Multi-chamber pacemakers deliver pacing pulses to multiple chambers. For example, dual-chamber pacemakers deliver pacing pulses to two heart chambers, e.g., the right atrium and the right ventricle, or the left and right ventricles. In a bi-ventricular pacing system, for example, a right ventricular pacing lead is positioned in the right ventricle of the heart and a left ventricular pacing lead is positioned via the coronary sinus in a cardiac vein, such as the middle or great cardiac vein. These pacing leads sense electrical activity that may be indicative of cardiac activity, such as ventricular contraction. The pacing leads also supply pacing pulses, i.e., electrical impulses that cause the heart to contract.
The timing of pacing pulses is often important. Many patients benefit from having chambers paced in a particular order with a delay between the respective pacing pulses. For example, in dual-chamber pacing, the length of time between an atrial sensed or atrial paced event and the delivery of a ventricular pacing pulse is known as the atrioventricular (AV) interval or AV delay. The optimal AV delay varies from patient to patient and may be determined using a number of techniques for evaluating cardiac performance, i.e., the efficiency of the heart as a pump. Assessments of cardiac performance may also be used in multisite pacing optimization.
Early cardiac performance assessment techniques focused on evaluating the performance of the right ventricle to realize a physiologic rate responsive function. With the increasing use of pacemakers to treat heart failure, however, it has become desirable to evaluate the performance of the left ventricle as well. Assessment of left ventricular performance is useful for monitoring the progression of heart disease, as well as for automatically driving electrical and drug therapies.
Some approaches have proposed estimating left ventricular performance based on measurements of right ventricular performance. For example, one commonly used parameter for evaluating cardiac performance is dynamic (or relative) pressure, dP/dt max, which is used to estimate contractility. In a normal heart, right ventricular dynamic pressure provides a reasonably accurate estimate of left ventricular dynamic pressure, but only for purposes of assessing contractility variations. Right ventricular dynamic pressure cannot be used to estimate absolute values of left ventricular dynamic pressure. In a failing heart, for example, the right ventricle may be normal and the left ventricle may be dilated, in which case the dynamics of dP/dt max in the right ventricle may not be the same as dP/dt max in the left ventricle. In addition, long-term reliability of implanted pressure sensors for measuring dynamic pressure has not yet been determined. Several months after implantation, fibrosis around the lead encapsulates the flexible membrane of the pressure sensor. This encapsulation may adversely affect the long-term reliability of the pressure sensor.
Right ventricular performance can also be assessed, for example, by estimating right ventricular stroke volume and pre-ejection interval (PEI) based on changes in impedance. In patients with heart failure, however, the heart can have a normal right ventricle and a dilated left ventricle. As a result, right ventricular performance may not be a reliable indicator of left ventricular performance.
Left ventricular performance can also be estimated by measuring endocardial acceleration. In particular, the peak endocardial acceleration (PEA) measures the amplitude of the first heart sound (FHS) as endocardially detected by a microaccelerometer in the tip of the pacing lead. It is well known that the first heart sound is affected both by left ventricular contractility and by the P-R interval. Consequently, an increase in the PEA may be attributable to an increase in contractility or to a decrease of the P-R interval. In other words, an increased amplitude of the first heart sound can indicate either good performance, i.e., increased contractility, or an AV delay that is too short, producing a short P-R interval. Amplitude assessment alone is therefore insufficient to conclusively evaluate myocardial performance.
Cardiac performance may also be evaluated by analyzing the timing of the first and second heart sounds. The first heart sound corresponds to the onset of ventricular systole, while the second heart sound corresponds to the onset of ventricular diastole. The first and second heart sounds can be detected using a variety of techniques, including phonocardiography, seismocardiography, and echocardiography. Out of these techniques, echocardiography is the most commonly used, but this technique requires the use of devices external to the patient.
Echocardiography may be used to obtain an index known as the myocardial performance index (MPI). The MPI is a mechanical index based on assessment of systolic and diastolic time intervals, namely, isovolumetric contraction time (ICT), isovolumetric relaxation time (IRT), and ejection time (ET). ICT is defined as the interval of left ventricular isovolumetric contraction, beginning at the end of diastole and ending at the beginning of systole. IRT is defined as the interval beginning at the end of systole and ending at the beginning of diastole. ET is the duration of systole.
Because the MPI is obtained via echocardiography, however, it is difficult to obtain the MPI using implantable devices. Generally, the MPI is obtained using devices external to the patient, limiting the ability to measure the MPI when the patient is not located at a facility with the appropriate equipment. Moreover, it is difficult to evaluate myocardial performance on a beat-by-beat basis using existing echocardiography techniques.
Multiple-chamber pacing systems are known in the art, including systems that pace and sense the right ventricle and the left ventricle. In addition, techniques associated with evaluating cardiac performance are known in the art. Table 1 lists patents that disclose pacemakers that use mechanical detection methods, such as heart sounds and accelerometers, to control pacemaker timing.
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the techniques of the present invention.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art with respect to cardiac pacemakers in general, and myocardial performance assessment in particular. These problems include, for example, difficulties in assessing myocardial performance on a beat-by-beat basis and the need for reliable evaluation of left ventricular performance. Various embodiments of the present invention have the object of solving at least one of the foregoing problems.
For example, it is an object of the present invention to assess myocardial performance using a combination of electrical and mechanical criteria. In particular, various embodiments of the invention assess myocardial performance by determining a QT interval based on electrogram (EGM) readings and by detecting the first and second heart sounds. The QT interval and the timing of the first and second heart sounds can then be used to evaluate certain parameters relating to myocardial performance.
In addition, it is object of the present invention to use these electrical and mechanical criteria to automatically drive therapies. For instance, the myocardial performance parameters obtained from the QT interval and from the timing of the first and second heart sounds may be used to optimize the AV delay and to optimize multisite pacing.
Some embodiments of the invention include one or more of the following features and advantages: (a) measuring myocardial performance using mechanical and electrical time intervals; (b) using T-wave and heart sounds to detect and assess myocardial performance based on ventricular pacing events; (c) using an external sound or vibration sensor to detect first and second heart sounds; (d) using internal implantable acceleration, vibration, or other sound sensors to detect acoustic signals emitted by the heart; (e) estimating myocardial performance using determined QT intervals and using the timing of the first and second heart sounds to calculate isovolumetric contraction time (ICT) and ejection time (ET) and the ratios ICT/QT and ET/QT; (f) analyzing heart sounds based on timing rather than signal amplitude; (g) automatically assessing ICT; (h) automatically estimating ET; and (i) estimating relaxation time.
Various embodiments of the present invention include methods and apparatuses for configuring heart failure pacemaker parameters based on measured QT intervals and first and second heart sounds, which correspond to mitral and aortic valve closures, respectively. Heart sounds may be detected using various internal and external techniques known in the art, including, but not limited to, accelerometers, microphones, piezoelectric sensors and transducers, and the like.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.